DESCRIPTION: The working hypothesis driving this proposal is that there are one or two anion binding sites within the pore and that the best probes of the pore are permeant anions themselves. Experimental methods combine expression of CFTR in Xenopus oocytes with single channel and macroscopic recording. In some cases, L-cells which stably express CFTR may be used (these cells will be provided by Dr. R. Bridges). Single channel recording is critical to confirm changes in conductance and mole fraction effects while permeation ratios can be easily determined from macroscopic currents. Controls will be in the form of non-CFTR expressing oocytes and oocytes expressing CFTR protein, but have not been stimulated to express current. The endogenous IClCa found in oocytes will be reduced by replacing extracellular Ca++ with Ba++. This proposal is focused on anion permeation independent of CFTR gating issues. These techniques will be used to develop a model of anion permeation for CFTR.